Nitrogen is often the rate-limiting element in plant growth, and all field crops have a fundamental dependence on exogenous nitrogen sources.
According to a recent study published in Field Crops Research (Volume 100, Issues 2-3, 1 Feb. 2007, Pages 210-217), Nitrogenous fertilizer, which is usually supplied as ammonium nitrate, potassium nitrate, or urea, typically accounts for 40% of the costs associated with crops, such as corn and wheat in intensive agriculture.
Improving nitrogen use efficiency of crop plants is an important goal towards reducing input costs and reducing the environmental consequences of intensive nitrogen fertilization on the environment. Increased efficiency of nitrogen use by plants should enable the production of higher yields with existing fertilizer inputs and/or enable existing yields of crops to be obtained with lower fertilizer input, or better yields on soils of poorer quality. Also, higher amounts of proteins in the crops could also be produced more cost-effectively.
Plants have a number of means to cope with nutrient deficiencies, such as poor nitrogen availability. One important mechanism senses nitrogen availability in the soil and respond accordingly by modulating gene expression while a second mechanism is to sequester or store nitrogen in times of abundance to be used later. The nitrogen sensing mechanism relies on regulated gene expression and enables rapid physiological and metabolic responses to changes in the supply of inorganic nitrogen in the soil by adjusting nitrogen uptake, reduction, partitioning, remobilization and transport in response to changing environmental conditions. Nitrate acts as a signal to initiate a number of responses that serve to reprogram plant metabolism, physiology and development (Redinbaugh et al. (1991) Physiol. Plant. 82, 640-650; Forde (2002) Annual Review of Plant Biology 53, 203-224). Nitrogen-inducible gene expression has been characterized for a number of genes in some detail. These include nitrate reductase, nitrite reductase, 6-phosphoglucante dehydrogenase, and nitrate and ammonium transporters (Redinbaugh et al. (1991) Physiol. Plant. 82, 640-650; Huber et al. (1994) Plant Physiol 106, 1667-1674; Hwang et al. (1997) Plant Physiol. 113, 853-862; Redinbaugh et al. (1998) Plant Science 134, 129-140; Gazzarrini et al. (1999) Plant Cell 11, 937-948; Glass et al. (2002) J. Exp. Bot. 53, 855-864; Okamoto et al. (2003) Plant Cell Physiol. 44, 304-317).
In the fields of agriculture and forestry, efforts are constantly being made to produce plants with an increased growth potential in order to feed the ever-increasing world population and to guarantee the supply of reproducible raw materials. There is a need for methods of increasing nitrogen use efficiency in plants, which leads to better growth potential and more biomass. This is done conventionally through plant breeding. The breeding process is, however, both time-consuming and labor-intensive. Furthermore, appropriate breeding programs must be performed for each relevant plant species. In addition, although great progresses that have been made about nitrogen utilization and the components involved in nitrogen use efficiency, such as nitrogen uptake, nitrogen assimilation and nitrogen partitioning or remobilization, much is still unknown about many of these complex interactions. Therefore, there is a continuing need for generally applicable processes that improve forest or agricultural plant growth to suit particular needs depending on specific environmental conditions. For example, genes that confer tolerance to growth on low nitrogen supply are valuable product prototypes for manipulating nitrogen use efficiency in plants (Good et al., 2004). One strategy to achieve such desirable traits involves genetic manipulation of plant characteristics through the introduction of exogenous nucleic acids conferring increased efficiency of nitrogen use by plants, which in turn should enable the production of higher yields with existing fertilizer inputs and/or enable existing yields of crops to be obtained with lower fertilizer input, or better yields on soils of poorer quality. Such approaches have the advantage of not usually being limited to one plant species, but instead being transferable among plant species. The present invention relates to a method for increasing growth potential, and/or increasing levels of nitrogen use efficiency in plants, characterized by expression of recombinant DNA molecules stably integrated into the plant genome